Fixed TypeError and standardized `num_topics` argument in `LdaModel.show_topics`

Line 678 of gensim/models/ldamodel.py threw a TypeError under Python3+, because the single slash division syntax returns floats, which cannot be used as indices. I changed this line to use floor division instead. This does not break Python2 compatibility.

Also solved Issue #115 by changing the `topics` argument of `LdaModel.show_topics` method to `num_topics`.
changed `topn` argument to `num_words` 

This brings the `LdaModel.print_topics` and `LdaModel.show_topics` in line with the equivalent `LsiModel` methods.
improve matutils docs
increase max word2vec sentence length to 10k
* was: max 1k tokens per sentence (rest is ignored)
improve docs
Merge pull request #1 from piskvorky/develop

catchup develop
'sample' parameter for downsampling frequent-words
simplify subsampling logic
bump version: 0.10.0
Merge branch 'release-0.10.0' into develop
word2vec: don't call random_sample when sample prob >= 1
Merge pull request #2 from piskvorky/word2vec_sample

Simplify subsampling logic
Merge remote-tracking branch 'upstream/develop' into develop
Merge pull request #201 from Tiepies/develop

Fixed TypeError and standardized `num_topics` argument in `LdaModel.show...
Merge pull request #206 from gojomo/develop

[word2vec] 'sample' parameter, for frequent-word down-sampling
update CHANGELOG with new pull requests